Currently, as a bonding wire for a semiconductor device connecting between electrodes on a semiconductor device and outer leads (hereinafter referred to as a “bonding wire”), thin wires with a wire diameter of about 15 to 50 μm are mainly being used. A method for bonding the bonding wire is generally a thermal compressive bonding technique with the aid of ultrasound, which uses a general-purpose bonder, a capillary tool used for bonding by passing the bonding wire therethrough, and the like. A bonding process of a bonding wire is carried out by heating and melting a tip of wire by arc heat input to form a ball (FAB: free air ball) through surface tension; crimp-bonding the ball part onto an electrode of the semiconductor device heated within a range of 150 to 300° C. (hereinafter referred to as “ball bonding”); forming a loop; and finally crimp-bonding a wire part onto an electrode of the outer lead (hereinafter referred to as “wedge bonding”). As bonding counterparts of the bonding wire, an electrode structure in which an alloy mainly containing Al is formed as a film on a Si substrate is used for the electrode on the semiconductor device, whereas an electrode structure plated with Ag or Pd is used for the electrode of the outer lead.
Au, which has been mainly used as a material of the bonding wire, has been being replaced with Cu mainly for LSI use. On the background of recent proliferation of electric vehicles and hybrid vehicles, needs for replacing Au with Cu are increasing also in on-vehicle device use.
As for a Cu bonding wire, there has been proposed a wire using high-purity Cu (purity: 99.99% by mass or more) (for example, Patent Literature 1). Cu has the drawback of being more susceptible to oxidation than Au and has problems in that bonding reliability, ball formability and wedge bondability are inferior. As a method for preventing surface oxidation of a Cu bonding wire, there has been proposed a structure in which a surface of Cu core material is coated with a metal such as Au, Ag, Pt, Pd, Ni, Co, Cr and Ti (Patent Literature 2). There has been also proposed a structure in which a surface of Cu core material is coated with Pd and a surface thereof is coated with Au, Ag, Cu or an alloy thereof (Patent Literature 3).